Powder metallurgy comprises the use of metal powders to form high-integrity, often fully-dense metal articles. It encompasses a number of very diverse metal fabrication techniques for the economical production of complex, near-net-shape articles. Examples of powder metallurgy fabrication techniques include extrusion, injection molding, compression molding, powder rolling, blow molding, laser forming, isostatic pressing, and spray forming. Powder metallurgy fabrication techniques offer several desirable features including the ability to easily produce graded structures, impregnate porous preforms, fabricate a dispersion of second phase particles in a parent matrix, and produce non-equilibrium phases and structures. While a number of materials can be formed using powder metallurgy techniques, highly-reactive metals are incompatible with current processing practices. Processing the reactive metals according to the powder metallurgy techniques known in the art typically results in metal articles containing unacceptably-high impurity concentrations. The presence of these impurities, particularly carbon, oxygen, and nitrogen, severely degrades the mechanical properties of the resultant articles. While alternative forming methods such as machining and casting exist, in many instances the alternatives are prohibitively expensive or produce components with unacceptable material properties. Therefore, the alternative forming methods have little value outside of niche markets.
Current titanium metal injection molding (MIM) practices provide excellent examples of powder metallurgy limitations. Titanium exhibits an amazing combination of properties; it is extremely lightweight, exceptionally resistant to corrosion, very strong and stiff, and resistant to creep and fatigue. Most powder metallurgy techniques, including MIM, involve mixing a metal powder with a primarily-polymeric or -aqueous binder, forming the shape of the metal article, heating to remove the binder, and then sintering at high temperature. However, titanium readily reacts with oxygen, carbon, and nitrogen at elevated temperatures, i.e. during binder burn-out and sintering, and loses many of its desirable properties. Consequently, titanium is generally incompatible with current MIM processes in applications calling for the mechanical properties of the contaminant-free metal.
Development of a binder system that is compatible with reactive metals appears to be the key technical barrier to making powder metallurgy techniques widely applicable and valuable across a broad range of materials and markets. Thus, a need for both a binder system and a method of forming metal articles exists for powder metallurgy of highly-reactive metals and metal alloys.